LTE-Advanced (LTE-A) system improves spectrum efficiency by utilizing a diverse set of base stations deployed in a heterogeneous network topology. Using a mixture of macro, pico, femto and relay base stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband user experience. In a heterogeneous network (HetNet), smarter resource coordination among base stations, better base station selection strategies and more advance techniques for efficient interference management can provide substantial gains in throughput and user experience as compared to a conventional homogeneous network. For example, coordinated multiple points (CoMP), also known as multi-BS/site MIMO, is used to enhance the performance of cell-edge UEs in LTE-Advanced systems.
In 3GPP RAN1#65, the issue of downlink control capacity was first discussed for CoMP scenario 4, where both macrocell base station and remote radio heads (RRH) inside the macrocell coverage share the same physical cell ID. In 3GPP RAN1#66, it was agreed as a working assumption to have a new physical control channel inside the region of legacy physical downlink shared channel (PDSCH). The main benefits to have this new physical control channel are for the better support of HetNet, CoMP, and MU-MIMO. In 3GPP RAN1#68, it was agreed that the enhanced physical downlink control channel (ePDCCH) spans both first and second slots in the region of legacy PDSCH.
Various proposals have been made related to the design of ePDCCH. The following is a list of such examples: R1-14124, “Multiplexing e-PDCCH with PDSCH” by Qualcomm, RAN1#67, November 2011; R1-114240, “Performance evaluation of enhanced control channel based on UE-specific reference signaling” by Samsung, RAN1#67, November 2011; R1-114396, “DM-RS based distributed and localized e-PDCCH structure” by Samsung, RAN1#67, November 2011; R1-114302, “DM-RS design for e-PDCCH in Rel-11” by NTT DoCoMo, Ran1#67, November 2011; and R1-120076, “On reference signal design for enhanced control channels” by Ericsson, RAN1#68, February 2012.
Based on ePDCCH design spanning in both first and second slots in the region of legacy PDSCH, it is desirable to design the physical structure of ePDCCH to support both distributed and localized transmission to exploit either diversity or beamforming gain. Furthermore, at most four antenna ports of UE-specific reference signals are used for ePDCCH. It is also desirable to know how to utilize the UE-specific reference signals for both distributed and localized transmission to exploit either diversity or beamforming gain in ePDCCH.